David Cameron has rejected criticism that the Conservatives' election strategy has been too timid, amid growing fears among senior Tories that they will struggle to win an overall Commons majority.

As party members gathered in Brighton for their last conference before the general election, Cameron insisted his policies were "bold and radical" and offered "big change".

His comments came as the party unveiled its election campaign slogan – Vote for Change – and pledged "immediate and real" action in six key areas: dealing with the budget deficit; boosting enterprise; shoring up families; backing the NHS; raising standards in schools; and cleaning up politics.

Cameron has faced questions from senior Tories as to why he has not yet "sealed the deal" with the electorate, despite Britain suffering the worst recession in 60 years and renewed questions over Gordon Brown's character and fitness for office.

A series of polls have shown the Conservative lead over Labour narrowing from around 20% to as little as five points, and the Tory leadership is holding emergency meetings to try to revive their campaign. In a sign of Tory fears that they will struggle to win a clear victory in the election, Cameron has established a team to plan for a hung parliament.

Defending his strategy in a video posted on his WebCameron blog today, Cameron said: "I defy anyone to look at our plans and call them timid, because the truth is they cannot be timid if we're to confront and defeat these problems.

"So we have made our choice. The Conservative party is a modern party and it's a bold, radical party – and that's the way it's going to stay."

He went on: "With Gordon Brown this country is going in the wrong direction and we need big changes to turn things around. And frankly there is an urgency to this work. We need change in our economy, backing aspiration and opportunity for all. Britain has been crying out for a modern alternative to this government that's failed so badly ... We have answered that call."

The shadow chancellor, George Osborne, admitted the Tory leadership had been making it "too easy" for Labour recently.

"Since the New Year, we are the party that has been setting out the new policy ideas that will change Britain," he wrote in the Times. "Perhaps that has made it too easy for the Labour party to simply attack us while escaping scrutiny themselves."

Osborne is due to tell the conference in Brighton today that a Conservative government would act quickly to cut business tax in a bid to rejuvenate the economy. He is expected to pledge to rebuild the "solid economic foundations of a Britain that works for all".

"Our country stands at one of those forks we come across as we travel the roads of our history – and we have to make our choice," he will say.

"We can either continue down a path of decline and fall, a path with rising debts, higher interest rates, ever rising taxes and high unemployment. Or we can change direction – tell the difficult truths, put debt and taxes back on a downwards trajectory and create the solid economic foundations of a Britain that works for all. That is the Conservative path."

The shadow foreign secretary, William Hague, made a rallying cry to the party faithful in his speech to the conference. He accused Brown of "dithering and vacillating" over when to call a general election, adding that only the Conservatives had the vision to put the UK back on its feet.

"I believe the choice for Britain is as stark as this: it is change or ruin," Hague said.